autosave: set frequency of saving commands

Set or disable autosave, which saves the XSPEC environment to a file periodically.

Syntax:           autosave <option>

where <option> is either off or a non-zero positive integer N. If the option is off, then auto-saving is disabled.If the option is N, the XSPEC environment is saved every N commands. The saving of the environment is equivalent to the command

XSPEC12>save all xautosav.xcm .,

i.e. both the file and model information is saved to the file xautosav.xcm, placed in the directory ~/.xspec/cache.  Thus in case of an unexpected crash, the state of XSPEC before the crash can be restored with the command source xautosav.xcm. The default value for the auto-save option is 1.